During the production of oil and gas, water is the most part volume waste stream. Produced liquid may contain dispersed and dissolved hydrocarbons, dissolved organics and dissolved inorganic compounds. In addition, it may also contain a significant amount of suspended solids of different origins (formation, precipitation, corrosion, etc.) and thus, different in nature (sand, clay, carbonates, sulphates, other.).

Before water can be reuse, it must be treated to remove some of the contaminants listed above. Typically, this produced water has been separated from the production stream in multistage separation drums.

If the produced water is to be discharged, dispersed hydrocarbons must be reduced to comply with the environmental regulations of the region. Currently, dispersed oil has been separated by gravitational processes ranging from settling and flotation to deoiling hydrocyclones, and in extreme cases centrifugation.

If the pretreated water is to be discharged into disposal wells or re-injected into the formation, suspended solids need to be reduced to prevent potential blockage of the receiving medium. In this case, oil is removed from the process stream by one or more of the methods mentioned above followed by a desanding step. This desanding has been accomplished with filtration in the past and desanding hydrocyclones more recently. In special cases, solids and oil have been removed in a single step and by deep bed filtration or liquid/solids separate centrifuges. Recently, cross flow membrane filtration has been try ,and get success to solve problem

The gravity settling section normally takes up the main part space in the separator.the size depends upon the physical properties of the fluids being separated, the volume of these fluids, the residence time required to achieve gravity settling, and if or not there are separator internals to assist the gravity separation.